1. Field of the Invention
This invention relates generally to gradient index (GRIN) lenses, and specifically to the use of positive and negative Abbe number radial GRIN rod lenses as optical relays in endoscopes, borescopes, and similar type instruments.
2. Description of the Related Art
Endoscopes, borescopes, and similar type optical instruments, hereinafer referred to as "remote viewing scopes", are well known in the art for their use in viewing, through relatively small openings, body cavities and industrial vessels, for example, where direct viewing is not possible. A typical early scope instrument is described by Hopkins in U.S. Pat. No. 3,257,902. It employs a series of rod lenses forming an objective system and a relay system for forming and relaying the image of an object down a narrow tube to a camera.
Systems of this type typically undercorrect axial chromatic aberration due to the convex, or positive, refracting surfaces of the lenses which are necessary to converge light and form real images. The aberration appears as a halo around the image because the blue light components of the image are focused closer to the lens than the red components along the optical axis. Ideally, an image formed in white light would have a common focus for all wavelengths or colors and thus be a clear and true representation of the object being viewed.
Since classical optical elements having negative power, i.e. concave refracting surfaces, characteristically overcorrect axial color by focusing the red components closer to the lens than the blue components, axial chromatic aberration in the image formed by a Hopkins-type scope is correctable by appropriately using additional negative power lenses in the device. This approach, however, adds to complexity and cost, especially in view of the frequency of use and sterilization requirements of modern devices of this type. Leiner and Prescott, in Applied Optics 22, 3 p 383 (1983) also point to difficulties in manufacturing classical lenses of the required size and surface contours for use in scope-type instruments.
Around 1970, Nippon Sheet Glass (NSG) developed solid gradient index rod lenses for use in scope-type optical systems. An advantage of the NSG GRIN rods was their flat refracting surfaces. In addition, it was realized that light rays propagating through the lens follow a sinusoidal path having a defined period when the index gradient profile varies essentially parabolically as a function of the lens radius. The rod lens could then be cut to a design length corresponding to the periodicity of the light propagation path to achieve desired imaging results. Axial and radial gradient index lenses are now well known in the art. Moore et al. in U.S. Pat. No. 3,729,253, describe gradient index properties and cite references to the design and manufacture of GRIN rods used, for example, as image relays.
An inherent problem of all conventional lenses is the chromatic aberration created by dispersion of the lens material itself. Optical dispersion is the change in the index of refraction as a function of the wavelength of the light passing through the lens. In general, the refractive index of a material is larger in shorter wavelengths than in longer wavelengths. Thus, the index of refraction in blue light is greater than in red light. Positive dispersion, therefore, is responsible for undercorrected axial color in optical systems. Similarly, dispersion of a GRIN lens also contributes to axial chromatic aberration.
Thus, there is a need for GRIN components which characteristically overcorrect axial color, and which can be used to simplify the optical systems of scope-type instruments for color corrected imaging. It is therefore an object of this invention to use negative dispersion GRIN lenses in the optical systems of endoscopes and similar type instruments to reduce or eliminate axial chromatic aberration in the image.
It is a further object of the present invention to use negative dispersion GRIN rods in integral and/or optical combination with conventional optical components to reduce or eliminate spherical aberration and coma in addition to axial chromatic aberration in the image.
It is another object of this invention to use radial GRIN lenses in optical and/or integral combination with diffractive or holographic optical elements for axial chromatic aberration correction.
It is a still further object of the present invention to use a plurality of radial GRIN rod lens segments or sections in an endoscope, or like optical device, to give such devices a degree of physical flexibility in addition to corrected imaging capability.